This invention relates to the detection of cracks in manufactured parts. More particularly, this invention relates to the detection of surface cracks in parts made of materials having a highly reflective surface and a low emissivity.
Manufacturers are often forced to optimize the design of parts that they manufacture in an effort to increase the efficiency of devices employing the parts. To that end, the design of the parts are optimized through the use of lighter weight materials and through the elimination of material that does not strengthen the part. The design optimization process assumes that the part is manufactured to exacting specifications.
Oftentimes, the optimized part includes complex geometric and curved surfaces which need to be manufactured. Manufacturers must identify parts that fail to meet the specifications in an efficient manner to control manufacturing costs. To reduce weight, manufacturers employ lighter weight materials such as aluminum, titanium and silicon which have a high reflectivity and a low emissivity. In order for these parts to perform as intended, the parts must be manufactured without surface irregularities such as cracks, pits, scratches and holes. If the surface irregularities are not found, the part may fail early which can be problematic if the part is critical to the safe operation of the device.
For example, aircraft manufactures currently manufacture complex geometric and curved aluminum stringers which can be 110 feet in length and 12 inches wide. These stringers are considered to be a critical structure of aircraft wings. After being manufactured, the stringer is transferred to a liquid penetrant inspection station for quality control. A liquid penetrant is applied to a surface of the part that is to be inspected. An operator inspects the part for surface irregularities. This technique is time consuming which increases the cost of the stringer and the time required to manufacture the stringer. The liquid penetrant that is applied to the stringer also contaminates the part.
Infrared crack detection involves applying a highly emissive material to a surface of the part to be inspected. The part is heated uniformly using a flash lamp or other suitable devices. The part absorbs or couples some of the energy contained in the flash. The coupled energy causes the entire part to radiate heat. If the part was manufactured perfectly, a thermal gradient for the part would vary in a continuous manner. Using an infrared camera, the thermal gradient of the part is measured. Abrupt discontinuities in the thermal gradient identify the surface irregularities such as surface cracks, pits, scratches and holes. This method fails to identify smaller cracks, scratches, and holes because the heat energy flows over the opening.
Unfortunately, the conventional infrared crack detection method cannot be used on aluminum having a thickness greater than approximately xe2x85x9xe2x80x3. Aluminum has relatively high heat dissipation characteristic. Therefore, if the aluminum part has a thickness, greater than xe2x85x9xe2x80x3, the heat generated by the flash lamp is quickly absorbed and dissipated by the aluminum. The thermal gradient does not adequately and reliably identify smaller surface irregularities such as cracks, pits, scratches and holes. If a higher energy source such as a laser is used, the emissive material is vaporized. If the highly emissive material is not used, an extremely high power laser is required to heat or couple with aluminum.
A detection system for identifying surface irregularities includes a part that is made of a material having a relatively high reflectivity in a relatively low emissivity. A laser source generates a laser beam. A scanning device scans the laser beam across a surface of the part. A surface irregularity radiates energy absorbed from said laser beam. An infrared receiver is directed at the surface of the part. The infrared receiver generates an infrared signal of the surface. A display that is connected to the infrared receiver displays the infrared signal to identify the surface irregularity.
In one feature of the invention, the scanning device directs the laser beam onto the part in an incident angle that is substantially perpendicular to the surface of the part.
In other features of the invention, the laser beam has a beam diameter that is approximately 50 microns or less. The infrared camera is oriented at a first angle relative to a line perpendicular to the surface of the part. The first angle is greater than 20 degrees and less than 30 degrees.
In still other features of the invention, a part transport device moves the part in a first direction. The scanning device scans the surface of the part in a second direction that is perpendicular to the first direction. An image processing module that is associated with the computer performs image processing on the infrared signal. A peak detection image processor associated with the computer identifies a surface irregularity by comparing the infrared signal generated by the infrared receiver through a threshold signal. The peak detection processor declares a surface irregularity when the infrared signal exceeds the threshold signal.
In yet another feature of the invention, the scanning device includes a first mirror and a second mirror. At least one of the first and second mirrors is rotatable and includes a plurality of facets for scanning the laser beam across the surface of the part.
Other objects, features and advantages will be readily apparent from the specification, the claims, and the drawings.